With the advent of OPEC, and the high cost and short supply of hydrocarbons, alternatives for existing vapor degreasing solvents has intensified.
The ordinary vapor degreasing solvents are normally chlorinated hydrocarbon ones, which meet the criteria of having no flash point and possessing good contaminant solvency and resusability through reclamation processes.
The prior art has utilized, as the basic vapor degreasing solvent, 1, 1, 1 trichloroethane or trichlorethylene for use in the ubiquitous vapor degreasing machine and operation. These solvents are ordinarily used in conjunction with a stabilizer which will extend the useful life of the vapor degreasing solvent.
In the vapor degreasing process, a non-flammable solvent is boiled to produce a vapor zone, the height of which is controlled by condensing coils. Cold work is introduced into the vapor, causes vapor condensation thereon, and the contaminant carried on the cold work, usually oil, grease or flux, is flushed off by the liquid solvent condensate. The contaminant, along with the condensate, is returned to the boiling sump of the vapor degreasing machine. This condensate, or distillate, then is revaporized to repeat the cycle of cleansing through condensation.
The work piece which is to be cleansed is held in the vapor zone until the temperature thereof reaches the vapor temperature within the vapor zone, at which time condensation stops. Vapor flushing is usually followed by pure distillate spray and/or liquid immersion. The cool, pure distillate reduces the temperature of the metal surface below the vapor temperature producing a second vapor condensation. When the work piece again reaches vapor temperature, it is withdrawn from the vapor zone, clean and dry.
The vapor degreasing solvent is used at its boiling point in order to produce the vapor zone necessary for vapor condensation and resultant cleaning.
Where 1, 1, 1 trichloroethane or trichlorethylene either alone or in conjunction with certain stabilizers to prolong the life thereof are utilized, serious drawbacks occur.
That is, during the degreasing operations, the degreasing solvent is adversely affected by the increasing amounts of contaminants finding their way into the boiling sump in that the boiling temperature of the solvent in the sump increases as the amount of contaminant increases. To compensate for this added contamination, solvent manufacturers add acid inhibitors or stabilizers in an effort to extend its vapor degreasing life.
When the temperature in the boiling sump of the vapor degreasing device reaches and exceeds a designated temperature range, normally signifying extensive contamination, depletion of the stabilizers is nearly complete and additional usage of the solvent is not recommended because of acidic breakdown and failure. For 1, 1, 1 trichloroethane this range is about 172.degree.-174.degree. F. whereas for trichlorethylene it is 195.degree.-198.degree. F.
Vapor degreasing handbooks recommend that vapor degreasers be shut down and the degreasing operation terminated to allow clean-out of the boiling sump once the boiling sump temperatures reach about 172.degree. F. for 1, 1, 1 trichloroethane and 195.degree. F. for trichlorethylene. The general criteria, measured in other terms for solvent rejuvenation, are when the boil sump specific gravity is between 1.21 and 1.33 or has an acid acceptance value of about 0.03-0.06, or wherein the pH value is between about 5.5-6.0.
In order to extend the life of the solvent by as much as 50%, and to reduce the boiling sump temperature, even with contamination present, and to provide a satisfactory vapor degreasing solvent of lower overall cost, it has been found that the addition of methylene chloride to 1, 1, 1 trichloroethane or trichlorethylene in an amount to reduce the initial boiling temperature of the resultant blend to about 110.degree. F. to 190.degree. F. achieves definite attributes, while alleviating many of the detriments found in prior art uses and methods of vapor degreasing using other solvents alone or with stabilizers to extend its useful life.
In the United States, environment protection regulations (EPA) dictate that a degreasing solvent may not contain more than 20% by volume of trichlorethylene. Thus, a solvent blend in accordance with this invention of about 20-10 volume percent of trichlorethylene and 80-90 volume percent methylene chloride is efficacious and better than trichlorethylene alone or methylene chloride alone.
In the conventional vapor degreasing process, there ideally exists about a 45.degree.-50.degree. F. temperature differential between the temperature of the inlet cooling water and the temperature of the degreasing vapors. Thus, where refrigerated or cooler temperatures are made available, a solvent blend of the invention using higher methylene chloride proportions may be utilized. In such cases, a preferred percentage of methylene chloride in the blends of the invention will be about 70-90 volume percent disregarding environmental regulations.
The lower boiling point of the resultant blends of the invention not only extends solvent life, but also lowers energy or heating requirements since the boiling point temperatures of the vapor degreasing solvents of the invention are lowered.